An electronic device in which a substrate constituting an electronic circuit is mounted on a heat sink and sealed with a mold resin together with a lead frame fixed to the heat sink, which is so-called a resin-sealed electronic device, is, for example, described in publication of Japanese Patent No. 3201277, Japanese Patent Application Publication No. 10-56126, and Japanese Patent Application Publication No. 2000-269415.
FIG. 13 shows an example of a manufacturing process of an electronic device. In the example of FIG. 13, a hybrid IC (hereinafter, HIC) substrate in which electronic parts are integrated and mounted on a wiring board is exemplarily employed as the substrate.
In S1 and S2, a silicon-based adhesive is printed on a heat sink. In S3, the HIC substrate on which an electronic circuit has been formed is mounted on the heat sink. In S4, the adhesive is hardened. In S5 and S6, the heat sink and the lead frame are fixed to each other. For example, the heat sink and the lead frame are fixed by such as crimping, welding, using adhesive tape, or the like.
In S7, a coating material made of a polyamide-based resin, which is a precursor of a polyimide-based resin, is deposited on the HIC substrate. The coating material is employed for restricting separation of the HIC substrate and the mold resin by alleviating thermal stress due to thermal expansion difference therebetween. The coating material is deposited along an outer end of the HIC substrate so as to surround bonding electrode pads. Then, the deposited coating material is heated once.
In S8, wire bonding is conducted, thereby to electrically connect the electrode pads and the lead frame. Although not shown in FIG. 13, a coating material having a viscosity lower than that of the coating material used in S7 is deposited over an entire surface of the HIC substrate after the wire bonding of S8, in order to reduce the separation.
In S9, the HIC substrate is sealed with a mold resin by transfer molding such that the heat sink and the lead frame are partly exposed. In S10, connecting portions of the lead frame are cut. In this way, the electronic device in which the substrate and the lead frame are fixed to the heat sink and sealed with the mold resin is manufactured.
In a case of an electronic device having a semiconductor chip or a small circuit board, it is generally known to form a seating for a substrate at a portion of a lead frame and use the seating as a heat sink. On the other hand, in a case of an electronic device having a relatively large substrate, such as the HIC substrate, the heat sink and the lead frame are generally provided as separate parts. Also, the coating material is required to restrict the separation of the substrate and the mold resin.